1. Field of the Invention
The present invention relates to a resin composition for a coating for an optical fiber, and to a coated optical fiber and an optical unit using such a resin composition. More specifically, the present invention relates to a resin composition for a coating for an optical fiber used for a secondary coating layer or a unit layer.
2. Description of the Related Art
Optical fiber cables are used as a transmission medium for large volumes of information, and broad band information communication networks using optical fiber cables continue to be constructed. Optical fibers include double layer coating structures in which a primary coating and a secondary coating formed from an ultraviolet light or radiation curing resin are provided on a glass waveguide, and single layer coating structures with a single coating layer.
The primary coating layer of a double layer coating structure is typically a flexible coating with a Young's modulus (tensile modulus) of 0.01 to 1.0 kg/mm2, whereas the secondary coating layer is typically a comparatively hard coating with a Young's modulus of 20 to 200 kg/mm2.
Furthermore, there are various configurations available for the use of optical fibers, including so-called unit configurations in which either a plurality of optical fibers comprising a coated glass waveguide, or a plurality of colored optical fibers provided with a colored layer as the outermost layer, are arranged either concentrically or within a flat plane, and are then integrated together via a radiation cured resin. A unit structure is a structure in which a plurality of optical fibers covered with either a single coating or a double coating are bundled together and wrapped with a curable coating (unit layer) of a radiation curable resin composition. Units comprising a plurality of integrated units are also possible. These unit configurations are an effective method of increasing the density of the optical fibers, and are widely used as structural components within optical fiber cables. The curable resin used in the primary coating layer, the secondary coating layer and the unit layer is known as a resin composition for a coating for an optical fiber.
A resin composition for a coating for an optical fiber for use as a secondary coating layer or a unit layer needs to display the following properties.
1. Rapid Processability
A low viscosity, and an ability to coat and cure the composition rapidly.
2. Rapid Curability
Satisfactory curing even with high speed processing (low dose) of several hundred m/minute, and an ability to obtain a cured product with the necessary Young's modulus.
3. Durability
The mechanical characteristics of a cured coating of the resin composition do not vary even when exposed to a variety of different environments (hot water, high humidity, high temperature, waterproofing mixture) over extended periods, and the durability of the optical fiber is good.
4. Temperature Characteristics
An optical fiber covered with a cured coating of the resin composition shows no variation in transmission characteristics or mechanical characteristics, even when used across a wide range of temperature conditions.
5. Toughness
A cured coating of the resin composition displays a good balance between Young's modulus and the breaking elongation.
6. Water Resistant Loss Characteristics
Even in cases in which the cable sheath is damaged by an accident following construction of the cable, and water enters the cable, the water causes no deterioration in the optical transmission characteristics of the optical fiber covered with a cured coating of the resin composition, even over long periods.
7. Storage
Even if the resin composition is stored at low temperature, no crystallizing occurs, meaning the life of the resin composition is very good.
Many different investigations have been conducted aimed at improving the above characteristics. For example, in Japanese Unexamined Patent Application, First Publication No. Sho 63-275619, combinations of a high molecular weight urethane acrylate with a number average molecular weight of 1000 to 15,000, and a low molecular weight urethane acrylate with a number average molecular weight of no more than 800 are disclosed as a method of improving the breaking elongation. Furthermore, in Japanese Unexamined Patent Application, First Publication No. Sho 63-168417, resin compositions for a coating for an optical fiber comprising an aliphatic radical polymerizable polymer with a polyether structure within the molecule, and a radical polymerizable polymer with a bisphenol A or bisphenol F type structure and a polyether structure within the molecule are disclosed as a method of improving the toughness and high speed curability of the composition. In addition, Japanese Unexamined Patent Application, First Publication No. Hei 5-163318 discloses optical fiber resin compositions comprising a urethane acrylate with ring structures within the molecule, in which the urethane linkage concentration within the composition is at least 2.0×10−3, and the coating produced on curing has a high Young's modulus (1500 MPa).
However recently, in addition to the characteristics described above, a further important characteristic has been identified as necessary. This new characteristic requires that for an optical fiber coated with a cured resin composition, in the case in which, for some reason or other, a scratch (or notch) forms in the surface of the coating layer formed from the resin composition, the cured composition displays sufficient tear resistance that the notch does not widen causing a major deterioration in the optical fiber characteristics during handling or extended installation (hereafter, in the present invention this characteristic is termed “notch resistance”), and also requires that when a unit needs to be deconstructed and the internal units or optical fibers removed, the unit layer can be peeled reliably at the desired position, producing a long and stable peeling in the longitudinal direction without splitting occurring partway along the peeling (hereafter, in the present invention this characteristic is termed “deconstructability”). In order to satisfy both of these requirements of notch resistance and deconstructability, it is important that the cured resin composition covering the optical fiber has an adequate tear strength, while also displaying a suitable Young's modulus and a suitable breaking elongation. However, in the case of the resin compositions for a coating for an optical fiber disclosed in the existing literature described above, no consideration was given to notch resistance to account for the possibility of a notch appearing due to heavy handling, nor to deconstructability, and consequently the resin compositions are not able to adequately satisfy these characteristics. Typically, the type of notch resistance described above is tested using the method disclosed in JIS K 6252-1993.